In the description which follows, reference is made primarily to the cornea of an eye, typically a human eye; however the invention is also applicable to measurements of lenses or any other curved objects which reflect visible or infrared light.
Optical coherence tomography (OCT) is a known technique for obtaining image information from within the eye using interferometric techniques. Longitudinal OCT has already been applied to measure the curvature and thickness of the cornea. Longitudinal OCT generates a B-scan image, which is a cross section image in the plane of lateral (angular direction) and depth in the tissue or axial distance from the instrument head. To proceed, a different ophthalmic instrument or a normal CCD camera is employed to acquire an en-face image of the cornea. A number of guiding radial lines are placed in this image over the cornea. OCT longitudinal cuts are subsequently acquired where the radial lines are placed, i.e. each longitudinal OCT contains one of such radial lines. Then the profiles of the epithelium and endothelium of the cornea in the B-scan images are used to determine the thickness and the curvature of the cornea. However, collecting a number of B-scans require time and the eye movements distort the profiles of the cornea collected.
The curvature of the cornea can also be inferred by using A-scans, which consist of line scans in the longitudinal direction, starting from a reference position and collecting A-scans of length just enough to acquire the peak corresponding to the cornea epithelium only. In this case only the curvature of the cornea is estimated. Again, as many A-scans are required, during the acquisition time, the axial eye movements lead to different axial positions of the cornea and so, to errors in the final profile.
OCT has also been reported as being capable of providing en-face images, as reported in “Coherence Imaging by Use of a Newton Rings Sampling Function” by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., Vol. 21, No. 21, (1996), pp. 1789-1791, “Simultaneous En-face Imaging of Two Layers in Human Retina” Opt. Letters, by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., 1997, vol. 22, No. 13, pp 1039-1041 and “En-face Coherence Imaging Using Galvanometer Scanner Modulation” by A. Gh. Podoleanu, G. M. Dobre, D. A. Jackson, Opt. Lett. 23, pp. 147-149, 1998, in “Transversal and Longitudinal Images from the Retina of the Living Eye Using Low Coherence Reflectometry”, by A. Gh. Podoleanu, Mauritius Seeger, George M. Dobre, David J. Webb, David A. Jackson and F. Fitzke, published in the Journal of Biomedical Optics, 3(1), pp. 12-20, 1998. En-face OCT allows generation of both B-scan and C-scan (or transversal) images. En-face OCT outputs a reflectivity profile along a trajectory transverse to the optics axis, which we will call it T-scan in the following.
FIG. 1 shows a B-scan image of the cornea made out of several T-scans, a transverse scans, collected at different depths, where the frame duration is the time required for depth advancement and FIG. 2 shows a C-scan image of the cornea by repeating T-scanning along the horizontal X-direction at several Y positions along the vertical coordinate, where the frame duration is determined by the time to advance the vertical position of the scanning beam. Both B-scan and C-scan images are affected by the eye movement. A C-scan, as is known in the art, is an image scan taken in a plane normal to the longitudinal direction (a C-scan has the orientation of an image taken by a microscope or a camera). Due to S/N ratio limitations, OCT cannot operate at video rate and consequently, the eye movements detrimentally affect the accuracy of data collected. The B-scan image is either axially stretched or compressed depending on the axial movement direction relative to the direction of frame scanning.
Similarly, due to movement, the C-scan image shows an external contour of the cornea which differs from a circle. FIG. 3a shows the external contour of the cornea as shown by C-scan images in the center of the cornea at different depths, a<b<c. FIG. 3b shows the external contour of the cornea at depth b while the eye moves axially towards the OCT instrument during the frame scanning duration. The contour in the first top half of the frame follows the correct contour of the circle of radius Rb while in the half frame at the bottom, that of a circle of larger radius, shown as Rc for the case when the eye moved axially to modify the equivalent depth from b to c. FIG. 3c shows the external contour of the cornea at depth b while the eye moves axially away from the OCT instrument. The contour in the first top half of the frame follows the correct contour of the circle of radius Rb while in the half frame at the bottom, that of a circle of smaller radius, shown as Ra, for the case when the eye moved axially to modify the equivalent depth from b to a. The central part of the image, which represents scattering points from inside the cornea volume confuses the border detection in the process of drawing a contour over the image. The C-scan image presents a transversal image through the Bowman layer, as the darker area between the external contour (epithelium) and the border of the bright scattering disk in the center of the image. This information has been discarded so far from quantitative evaluations.
As another disadvantage, a single B-scan OCT image displays the curvature along the azimuth angle cut chosen, no information exists in this image on the curvature along other azimuth angle orientations.
FIG. 2 shows an en-face OCT image collected with an OCT system as described in the paper “En-face OCT imaging of the anterior chamber”, by A. Gh. Podoleanu, J. A. Rogers, G. M. Dobre, R. Cucu, D. A. Jackson published in the SPIE proceedings, Vol. 4619, 2002, p. 240-243. The image was collected in 0.5 s and in the same time interval, the eye has moved axially towards the scanning head of the OCT and the shape of the external contour of the image deviates from a circle. If only an OCT longitudinal image would have been collected along the cut AA′ shown in FIG. 1, then the curvature on the left (about A) would have been less than the curvature of the profile shown on the right (about A′). Obviously, a B-scan OCT image would have come up distorted and provided an incorrect value for the cornea curvature. It would be more difficult on the individual B-scan image to distinguish the asymmetry. This image demonstrates that the asymmetry is easier visible in the C-scan than in the B-scan imaging, however so far C-scans have not been used to assess the cornea curvature.
Axial and lateral movements of the eye during the image acquisition manifest differently in the image depending on the scanning mode, B or C-scan.
Thus, a need exists for a better procedure to evaluate the cornea shape and reduce the effect of the eye movement when using OCT to determine the curvature of the cornea. The method and systems proposed are equally useful for stationary objects, providing the curvature of the object in one or fewer steps than required by the known technologies.
Different methods for measurement of the axial and lateral position of the eye are known, as described in the U.S. Pat. No. 7,113,818. They use position sensors or OCT systems. They are cumbersome, slow and usually, the methods for transversal movement or shift evaluation are different from the method and system to evaluate the axial position. If they are applied in robotics to evaluate the inclination of the object, then different systems implementing different technologies should be combined.
Therefore a need exists for a simpler and faster method and system to provide the axial distance of an object as well as its tilt, using the same and unique measuring technology.